Tumour filled with blood. Coloured scanning electron micrograph (SEM) of a malignant (cancerous) tumour filled with erythrocytes (red blood cells). This is a haemangiosarcoma, a rare form of cancer that arises from the cells that line blood vessels (vascular endothelial cells). It is most common in dogs, but can also occur in humans and cats. It does not normally present with symptoms until a late stage when it is incurable. Tumours frequently rupture causing the individual to bleed to death.